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Subject: Re: [PATCH 1/2] drm/tegra: Get ref for DP AUX channel, not its ddc
 adapter
From:   Lyude Paul <lyude@redhat.com>
To:     Thierry Reding <thierry.reding@gmail.com>
Cc:     linux-tegra@vger.kernel.org, dri-devel@lists.freedesktop.org,
        Thierry Reding <treding@nvidia.com>,
        Ville =?ISO-8859-1?Q?Syrj=E4l=E4?= 
        <ville.syrjala@linux.intel.com>,
        Jonathan Hunter <jonathanh@nvidia.com>,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        open list <linux-kernel@vger.kernel.org>
Date:   Tue, 27 Apr 2021 18:44:52 -0400
In-Reply-To: <YIZu4PD6BmlGYXuc@orome.fritz.box>
References: <20210423182146.185633-1-lyude@redhat.com>
         <20210423182146.185633-2-lyude@redhat.com>
         <YIZu4PD6BmlGYXuc@orome.fritz.box>
Organization: Red Hat
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On Mon, 2021-04-26 at 09:42 +0200, Thierry Reding wrote:
> On Fri, Apr 23, 2021 at 02:21:45PM -0400, Lyude Paul wrote:
> > While we're taking a reference of the DDC adapter for a DP AUX channel in
> > tegra_sor_probe() because we're going to be using that adapter with the
> > SOR, now that we've moved where AUX registration happens the actual device
> > structure for the DDC adapter isn't initialized yet. Which means that we
> > can't really take a reference from it to try to keep it around anymore.
> > 
> > This should be fine though, because we can just take a reference of its
> > parent instead.
> > 
> > Signed-off-by: Lyude Paul <lyude@redhat.com>
> > Fixes: 39c17ae60ea9 ("drm/tegra: Don't register DP AUX channels before
> > connectors")
> > Cc: Lyude Paul <lyude@redhat.com>
> > Cc: Thierry Reding <thierry.reding@gmail.com>
> > Cc: Jonathan Hunter <jonathanh@nvidia.com>
> > Cc: dri-devel@lists.freedesktop.org
> > Cc: linux-tegra@vger.kernel.org
> > ---
> >  drivers/gpu/drm/tegra/sor.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/tegra/sor.c b/drivers/gpu/drm/tegra/sor.c
> > index 7b88261f57bb..4e0e3a63e586 100644
> > --- a/drivers/gpu/drm/tegra/sor.c
> > +++ b/drivers/gpu/drm/tegra/sor.c
> > @@ -3739,11 +3739,11 @@ static int tegra_sor_probe(struct platform_device
> > *pdev)
> >                 if (!sor->aux)
> >                         return -EPROBE_DEFER;
> >  
> > -               if (get_device(&sor->aux->ddc.dev)) {
> > -                       if (try_module_get(sor->aux->ddc.owner))
> > +               if (get_device(sor->aux->dev)) {
> > +                       if (try_module_get(sor->aux->dev->driver->owner))
> >                                 sor->output.ddc = &sor->aux->ddc;
> >                         else
> > -                               put_device(&sor->aux->ddc.dev);
> > +                               put_device(sor->aux->dev);
> >                 }
> >         }
> 
> Unfortunately, I think it's a bit more subtle than that. The reason for
> this get_device()/try_module_get() dance was to mirror the behaviour of
> of_get_i2c_adapter_by_node() so that when we call i2c_put_adapter() in
> tegra_output_remove() we correctly decrease the reference count.
> 
> The above will increase the reference on the I2C adapter's parent while
> i2c_put_adapter() will then only decrease the reference on the I2C
> adapter, so I think effectively we'd be leaking a reference to the I2C
> adapter's parent.
> 
> Also, since we didn't take a reference on the I2C adapter explicitly,
> releasing that reference in tegra_output_remove() might free the I2C
> adapter too early.
> 
> I wonder if perhaps it'd be easier to get rid of the struct tegra_output
> abstraction altogether and push this down into the individual drivers,
> even if that means a bit more code duplication. That's not the kind of
> quick fix to resolve this current situation, so perhaps as a stop-gap we
> just need to sprinkle a few more conditionals throughout tegra_output
> code. We could, for example, avoid calling i2c_put_adapter() in
> tegra_output_remove() for the DisplayPort cases and instead manually
> release the reference to the I2C adapter's parent in tegra_sor_remove().
> On top of your patch above that /should/ fix things properly for now.

Alright - I will try to get to this tomorrow

> 
> Thierry

-- 
Cheers,
 Lyude Paul (she/her)
 Software Engineer at Red Hat